This invention relates to an air flow meter, and more particularly to a thermal flow meter for use in determining the flow rate of suction air in an internal combustion engine for automobiles.
There are various kinds of known systems for determining the flow rate of suction air in an internal combustion engine. Among these systems, a system using a thermal air flow meter is widely employed for the following reasons: A thermal air flow meter generally has a high responding capability, and it permits determining the mass flow rate of the air, so that the compensation of air pressure is not required. Such a system is known from U.S. Pat. No. 3,747,577, U.S. Pat. No. 3,750,632 and U.S. Pat. No. 3,824,966. This known thermal flow meter consists of a platinum wire having a diameter of 70-100 .mu.m provided in a suction pipe, which platinum wire serves as a velocity of flow detecting unit.
Besides the above system using a flow meter, a system is disclosed in U.S. Pat. No. 4,264,961 which uses a detecting unit consisting of a hollow ceramic support, a platinum wire wound around the support, and a layer of glass formed so as to cover the combination of the ceramic support and the platinum wire.
However, these thermal flow meters have the following problems. Namely, these flow meters are not capable of discriminating the direction, in which fluid flows in a tubular passage. Thus, even when fluid flows in a reverse direction in the passage, a signal which is identical with the signal generated when the fluid flows in a forward direction is outputted. When this flow meter is applied to an internal combustion engine, it is disposed between the air cleaner and the engine to determine the flow rate of the air sucked into the engine through the air cleaner. However, when the internal combustion engine is in a certain rotational condition, the air may be blown backward therefrom into the air cleaner in some cases. In such cases, the air blown backward is detected as a forward flow by the thermal flow meter, and a signal representative of a flow rate determined from the backwardly-blown air is outputted. Consequently, a measurement error occurs.